The invention relates to high density cathode materials for secondary batteries, obtained by the precipitation of heterogeneous metal bearing material, that is homogeneously doped with a nanoparticle metal oxide, metal halide, metal anion, or elemental metal component.
Compared to Ni—Cd and Ni-MH rechargeable batteries, Li-ion batteries boast an enhanced energy density, mainly due to their higher 3.6 V working voltage. Since their commercialization in 1991 by SONY, Li-ion batteries have seen their volumetric energy density increase continuously. This has been initially realized by optimizing cell design, accommodating more active electrode materials in a fixed volume cell. Later efforts concentrated on improving the energy density of the electrodes. Using a high density active electrode material is another way to achieve this goal. As LiCoO2 still continues to be used as positive electrode material for the majority of commercial Li-ion batteries, a highly dense variety of this material is in demand.
In WO2009/003573 such a high density LiCoO2 material has been disclosed. It provides a relatively coarse-grained electrochemically active LiCoO2 powder, without significant Li-excess, and having a D50 of more than 15 μm, a BET of less than 0.2 m2/g. The mentioned particle size is evidently a primary particle size, and the particles are neither agglomerated or coagulated, nor aggregated.
However, this material shows various limitations in a rechargeable lithium battery. One basic limitation originates from the surface area dilemma. Increasing the rate performance (i.e. high power) can be met by increasing the surface area because the solid-state lithium diffusion length can be decreased; which results in an improved rate performance. However, a high surface area increases the area where unwanted side reactions between electrolyte and charged cathode take place. These side reactions are the reason for poor safety, poor cycling stability at elevated voltage and of poor storage properties of charged cathodes at elevated temperature. Furthermore, high surface area materials tend to have a low packing density which reduces the volumetric energy density.
Recent findings have shown that doping LiCoO2 cathode materials with different elements, including but not limited to Mg, Ti, Zr, Cr, and Al, has yielded products with improved cycle life, stability, performance, and safety characteristics. The advantages of Ti doping for LiCoO2 have been mentioned in U.S. Pat. No. 6,277,521.
As for most cathode materials, their preparation often makes use of a particular transition metal precursor. The precursor can then be fired with a lithium source to prepare a cathode material. It is therefore important to prepare precursors that can be easily transformed into cathode materials. It is even more beneficial if the precursors can be easily doped with other elements and that the precursor can be used to directly prepare the cathode material without additional processing steps. In WO2009/074311 various methods for preparing cathode precursor material were discussed, amongst others precipitation, coprecipitation, spray drying, spray pyrolysis, physical mixing or blending, also using slurries. All of these methods have serious problems in achieving good homogeneous doping, especially for Ti doping using a material like nanoparticles of TiO2.
As initially said, an essential feature of LiCoO2 is its high density. The crystallographic density is higher than other cathode materials, namely 5.05 g/cm3, and LiCoO2 shows a good performance even if particles are relatively large and compact. The large and compact particles pack well and thus allow to achieve electrodes with high density. High density electrodes allow to insert a larger mass of active LiCoO2 into the confined space of a commercial cell. Thus a high density of LiCoO2 is directly related to a high volumetric density of the final commercial lithium battery. A preferred morphology to achieve high density are compact—mostly monolithic, non-agglomerated—particles. A typical particle size (D50) is at least 10 or even 15 μm, and typically it is less than 25 μm.
There are two major preparation routes to prepare such monolithic LiCoO2. In the first route a source of cobalt (like CO3O4) with relatively small particle is mixed with a source of lithium (like Li2CO3) and fired at sufficient high temperature with a sufficient excess of lithium. During sintering, the small CO3O4 particles sinter together and particles grow to the desired size distribution. In the second, alternatively route, relatively large and dense particles of a cobalt source are used. During sintering particles tend to sinter independently. There is a densification within a particle, but not much inter-particle sintering. A problem happens if applying these standard methods to prepare Ti doped LiCoO2.
The first method basically results in a failure. If a mixture of TiO2, small particles CO3O4 and a source of lithium is sintered, by an—to us unknown and surprising mechanism—inter-particle sintering is very much suppressed. As a result a high surface area LiCoO2 consisting of heavily agglomerated particles is achieved. The preferred morphology mentioned before is only achieved after applying unrealistically high sintering temperature or much larger Li excess. Much higher sintering temperature increases the cost significantly—equipment investment, live time and energy use. Much larger Li excess results in poor performance.
The second obvious method to prepare Ti doped LiCoO2 is as follows: a relatively dense cobalt precursor (like Co(OH)2 with large particles size, a source of lithium (like Li2CO3) and a source of titanium (like TiO2) are mixed, followed by sintering. In this case—where the TiO2 is not well distributed within the particle—we observe an inhomogeneous final product. The reason is that TiO2 has very poor mobility during sintering, so wherever in the mixture a few TiO2 particles are agglomerated the final LiCoO2 will show a region with much higher TiO2 concentration. As a result, the TiO2 doping with low doping level (0.1-0.5 mol %) is not efficient. At higher levels a benefit is observed, but because of the poor Ti mobility it is assumed that the inside of the LiCoO2 particles is basically free of Ti and the full benefit of Ti doping cannot be achieved.
A third method to prepare high density TiO2 doped LiCoO2 is a two step firing. In a first firing a LiCoO2 precursor with preferred morphology is prepared. This LiCoO2 precursor is mixed with TiO2, typically at least 0.75 mol % and less than 2 mol % (smaller doping levels are not efficient, the reason is the same as in the second method—any TiO2 agglomerate will cause a TiO2 enriched region resulting in a non-homogenious final LiCoO2). After sintering it is assumed that the LiCoO2 core is free of TiO2 and the full benefit of Ti doping is not achieved.
In US2007/0264573 A1, on the other hand, an aqueous solution of Mg carbonate, Al and Ti lactate solution is mixed with a Co hydroxide slurry, and after wet ball milling the slurry is spray-dried for granulation. These precursor granules are mixed with Li carbonate and sintered at 1000° C. to obtain a Li Co—Mg—Al—Ti oxide. Since it is generally known that such a spray-drying operation is carried out at a temperature below 120° C., and since Ti lactate is a fairly stable compound that will crystallize at such temperatures—since it will only disintegrate to form Ti dioxide at temperatures over 200° C.—the spray-dried precursor does not contain TiO2 in the form of nanoparticles being homogeneously distributed within the precursor.
It can further be mentioned that in CN1982219 A, a Li Co—Ni—Mn oxide doped with Al, Ti, Mg and/or Cr is obtained by co-deposition, whilst in CN101279771 A, a Mg, Al and/or Ti source are mixed in a cobalt nitrate solution which is precipitated as a doped cobalt hydroxide.
It is the scope of the present to provide for a manufacturing method for a cathode material having a high rate performance, showing high stability during extended cycling at high charge voltage, and having a particularly high pellet density. The high temperature storage properties are also to be improved.